Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. For the formation of the positive silver images, a latent image contained in an exposed photosensitive silver halide emulsion is developed and almost concurrently therewith, a soluble silver complex is obtained by reaction of a silver halide solvent with the unexposed and undeveloped silver halide of said emulsion. Preferably, the photosensitive silver halide emulsion is developed with a processing composition in a viscous condition which is spread between the photosensitive element comprising the silver halide emulsion and a print-receiving element comprising a suitable silver precipitating layer. The processing composition effects development of the latent image in the emulsion and, substantially contemporaneously therewith, forms a soluble silver complex, for example, a thiosulfate or thiocyanate, with undeveloped silver halide. This soluble silver complex is, at least in part, transported in the direction of the print-receiving element and the silver thereof is precipitated in the silver precipitating element to form a positive image thereon. Procedures of this description are disclosed, for example, in U.S. Pat. No. 2,543,181 issued to Edwin H. Land. See, also, Edwin H. Land, One Step photography Photographic Journal, Section A, pp. 7-15, January 1950.
Additive color reproduction may be produced by exposing a photosensitive silver halide emulsion through an additive color screen having filter media or screen elements each of an individual additive color, such as red or green or blue, and by viewing the reversed or positive silver image formed by transfer to a transparent print-receiving element through the same or a similar screen which is suitably registered with the reversed positive image carried by the print-receiving layer.
As examples of suitable film structures for employment in additive color photography, mention may be made of U.S. Pat. Nos. 2,861,885; 2,726,154; 2,944,894; 3,536,488; 3,615,427; 3,615,428; 3,615,429; 3,615,426, and 3,894,871.
In general, silver precipitating nuclei comprise a specific class of adjuncts well known in the art as adapted to effect catalytic reduction of solubilized silver halide specifically including heavy metals and heavy metal compounds such as the metals of Groups IB, IIB, IVA, VIA and VIII and the reaction products of Groups IB, IIB, IVA and VIII metals with elements of Group VIA, and my be effectively employed in the conventional concentrations traditionally employed in the art.
Widely used as silver precipitating agents are those disclosed in U.S. Pat. No. 2,698,237 and specifically the metallic sulfides and selenides, there detailed, there terms being understood to include the selenosulfides, the polysulfides, and the polyselenides. For best results it is preferred to employ sulfides whose solubility products in an aqueous medium at approximately 20.degree. C. vary between 10.sup.-.sup.23 and 10.sup.-.sup.49, and especially the salts of zinc. Also particularly suitable as precipitating agents are heavy metals such as silver, gold, platinum, palladium, etc., and in this category the noble metals illustrated are preferred and are generally provided in a matrix as colloidal particles.